Basalt flows of the upper Cenozoic Mount Taylor volcanic field in central New Mexico are divisible into four stratigraphic sequences that define the general petrologic progression: early undersaturated alkalic basalt, two sequences of relatively silicic alkalic basalt, and late olivine tholeiite. This progression is characterized by increasing SiO2 and decreasing K2O, TiO2, P2O5, Sr, U, and Th. Recent experimental data permit interpretation of such a progression in terms of fractionation of magma at progressively shallower levels in the upper mantle. In the western United States, Cenozoic basalts of such compositional diversity occur mainly along boundaries between major tectonic provinces, especially around the margins of the Basin and Range province and the Colorado Plateau.

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